posted on 2021-10-18, 17:06authored byYu Katayama, Ryoma Kubota, Reshma R. Rao, Jonathan Hwang, Livia Giordano, Asuka Morinaga, Takeou Okanishi, Hiroki Muroyama, Toshiaki Matsui, Yang Shao-Horn, Koichi Eguchi
Direct methanol fuel cells (DMFCs)
using alkaline electrolytes
are of interest due to the applicability of nonprecious metal-based
materials for electrocatalysts. However, the lack of understanding
of the methanol oxidation reaction (MOR) mechanism in alkaline media
hinders the development of active catalysts for the MOR. In this work,
ambient-pressure XPS and in situ surface-enhanced
infrared spectroscopy were performed on the Pt surface in order to
gain experimental insights into the reaction pathway for the MOR.
We present a comprehensive reaction mechanism for the MOR in alkaline
media and show that the MOR proceeds via two different
pathways depending on the electrode potential. We confirmed the formation
of partially hydrogenated CO adsorbates [HxCOad···(OH) (1 < x <
3)] via water and/or hydroxide ion-mediated dissociation
of methanol. The HxCOad···(OH)
species were further dehydrogenated to COad in the potential
range of 0.40–0.60 VRHE and subsequently oxidized
to CO2 by reactive OHad on the Pt surface at
0.65 VRHE (pathway I). Furthermore, H3C–Oad intermediates were observed at potentials higher than 0.9
VRHE, at which the MOR proceeds mainly via H3C–Oad instead of COad intermediates
(pathway II). The oxidation current related to this conversion from
H3C–Oad to CO2 (pathway II)
dominates the overall MOR current, suggesting that the H3C–Oad pathway could be one of the keys to enhancing
the MOR activity in an alkaline environment. Our findings pave the
way toward a design strategy for MOR electrocatalysts with improved
activity based on the experimental reaction mechanisms that have been
identified.